What Makes a “Classic?”

I’m not ashamed to admit that I really liked The Da Vinci Code. It was a page-turner, and I like history, art, puzzles, and Masons. But recently at a reputable bookstore, I saw a display of “Classics” that included Moby Dick, The Great Gatsby, and The Da Vinci Code.

My gut tells me that putting Dan Brown with Melville and Fitzgerald is ridiculous. But I also remember the first time I heard Nirvana on an Oldies-type station, I objected. Maybe my gut feeling is just telling me I’m getting old.

What are the requirements of something becoming a classic? Wildly popular? Taught in high schools? So deep that no one really understands it but is afraid to admit it?

How long does something have to exist before it’s labeled classic?

Also, I swear a few weeks ago, I saw Cliff’s Notes for The Da Vinci Code. But I’ve been having trouble discerning reality from my dreams lately, so I can’t confirm that.